Laser initiated deposition of films can be conveniently categorized as patterned or unpatterned processes wherein a laser beam imparts photon or thermal energy to reactants causing the reactants to dissociate and form a patterned or unpatterned layer on a substrate. Patterned laser depositions may be formed either by "direct writing" or projection patterning. In projection patterning an image is focused through a mask; whereas in "direct writing", a focused laser spot is translated along a substrate to form the desired pattern.
U.S. Pat. No. 4,340,617 to Deutsch et al. shows a "direct writing" laser photodeposition process for the direct generation of patterns of materials, such as metals or semiconductors, on substrates without the intermediate photolithographic steps required in conventional microelectronic fabrication. In the Deutsch et al. system, precursors, or reactants, in the gaseous phase are introduced into a reaction chamber in which a substrate is disposed. A laser beam is focused on, or adjacent to, the surface of the substrate upon which the pattern of material is to be deposited. One or more of the reactants absorbs a portion of the incident energy at a predetermined laser frequency, thereby effecting photodecomposition, or photolysis, of one or more of the reactants. For example, gaseous trimethylaluminum or dimethyl cadmium are dissociated to form aluminum or cadmium metal patterns on a substrate surface using a 257-nm laser beam.
The Deutsch et al. laser-induced deposition process is based on local-area photolytic reactions of gaseous precursors. Hanabusa et al..sup.(1) describe an alternate primarily pyrolytic deposition method. This method involves local heating of the substrate with a focused laser beam.
Although more difficult in practice, substantially photolytic reactions are often preferred since they can be sustained at low temperatures and are insensitive to substrate topography, thermal conductivity, or reflectivity changes. In particular, applications on thermally fragile or transparent substrates or membranes often require such nonthermal methods.
However, because of the general inaccessibility of the dissociative transitions of most stable precursor molecules, photolytic deposition has heretofore been limited to reactions excited in the UV at wavelengths shorter than those of simple commercially available visible light emitting cw lasers. It is highly preferable to use a visible laser for many applications because of the practical simplicity and reliability of visible light emitting lasers. A low power visible light process for depositing opaque, adherent film patterns is particularly desirable for certain applications, such as the repair of photomasks for optical lithography.
Additionally, a need exists for a photolytic deposition process capable of inducing photochemical deposition of ferromagnetic material on substrates using low power visible light. For example, chromium oxide is a ferromagnetic material extensively utilized in the fabrication of Surface Acoustic Wave (SAW) devices. Thin films of this material undergo irreversible transformation to the non-magnetic oxides of chromium during conventional high temperature deposition processing..sup.(2)(3)